kdb: Merge identical case statements in kdb_read()

The code that handles case 14 (down) and case 16 (up) has been copy and
pasted despite being byte-for-byte identical. Combine them.

Cc: stable@vger.kernel.org # Not a bug fix but it is needed for later bug fixes
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20240424-kgdb_read_refactor-v3-4-f236dbe9828d@linaro.org
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>